improve CI: test on minimum required version, run exist as service

[line-o]

• simpler setup with existdb docker container running as a service
• existdb startup time goes from 30 seconds down to 15 seconds by mounting an empty folder to autodeploy
• test also on the minimum required processor version v5.2.0 to ensure backwards compatibility
• remove caching setup as this is built-in to the newer actions
• remove steps to build and copy XARs to autodeploy as XAR installation is part of npm test
• update to latest version of gulp-exist (v4.3.2)